1. Field of the Invention
The present invention relates to a light source device using fluorescence emitted from a phosphor by excitation light, and in particular to a light source device emitting visible light of red, green and blue for use in an image display apparatus.
2. Description of Related Art
Recently, a projector as an image display apparatus that magnifies and projects various video images on a screen has been used widely. Such a projector has a function of condensing light emitted from a light source onto a spatial light modulation element such as a digital micromirror device (DMD) or a liquid crystal display element, and displaying exiting light from the spatial light modulation element modulated by video signals on a screen as color video images.
In order to obtain a bright and large-screen video image with the projector, conventionally, a high-brightness high-pressure mercury lamp has been used as a light source. However, when using the high-pressure mercury lamp as a light source, there are problems of a short lifetime of the light source, complex maintenance, etc.
For solving these problems, in place of the high-pressure mercury lamp, a light source device employing a solid light source such as a laser and a LED has been studied for use in an image display apparatus. The laser light source has a longer lifetime as compared with the high-pressure mercury lamp and has high directivity and light use efficiency owing to coherent light. Further, its monochromaticity makes it possible to secure a wide color reproduction range.
However, due to the high coherency of the laser light, a speckle noise is generated, which deteriorates image quality. Specifically, as to the laser light in wavelengths ranging from green to yellow where the human eye has high visibility, the decrease in image quality due to the speckle noise is a large problem. Further, although, in the LED light source, the speckle noise does not become a serious problem, a low light density of the light source due to a large light-emitting area makes it difficult to obtain a high-brightness image display apparatus.
Meanwhile, studies have been done to configure, as the solid light source other than the laser and the LED, a light source device that utilizes a light emitted from a phosphor excited by a light source such as a laser and a LED and to use the light source device in an image display apparatus. In the light source device using a phosphor, it is possible to obtain a small light-emitting area by using a laser light source capable of condensing light with high density as the excitation source. Further, even when the laser light source is used as the excitation source, a speckle noise is not generated because the fluorescence itself obtained by frequency conversion is incoherent light.
Such a light source device utilizing a phosphor is disclosed in JP 2004-341105 A and JP 2009-277516 A, for example. The devices disclosed in these documents are configured such that: a phosphor layer is disposed on a transparent disk-shaped base; excitation light is irradiated onto the phosphor layer; and fluorescence is extracted in a space on a side opposite to an excitation light source when seen from the base on which a phosphor is disposed.
However, since a large amount of fluorescence from the phosphor layer is emitted in a space on the same side as the excitation light source when seen from the base on which the phosphor is disposed, it is difficult to obtain a high-efficiency light source device by the aforementioned conventional configuration.
In contrast, for example, a configuration of the light source device shown in FIG. 10 can be considered for extracting fluorescence in the space on the same side as the excitation light source. In this light source device, a phosphor layer 101 emitting fluorescence by irradiation of excitation light is formed on one side surface of a base 100. The base 100 has a circular shape, and can be rotated with a rotation device 102. An excitation light source 103 is composed of a plurality of laser diodes emitting excitation light. The laser diode is a blue laser diode oscillating in the vicinity of a wavelength of about 445 nm.
Excitation light emitted from the excitation light source 103 is collimated by a collimator lens array 104, and enters a dichroic mirror 105. The dichroic mirror 105 is configured to transmit the excitation light from the excitation light source 103 and reflect fluorescence from the phosphor layer 101 generated by irradiation with the excitation light. Therefore, the excitation light having passed through the dichroic mirror 105 is condensed onto the phosphor layer 101 by a condenser lens 106.
A part of the excitation light irradiated onto the phosphor layer 101 is converted into fluorescence, and emitted to spaces on both sides when seen from the base 100. The fluorescence emitted in the space on the same side as the excitation light source 103 when seen from the base 100 is collimated by the condenser lens 106, reflected by the dichroic mirror 105, and passes through a dichroic mirror 110, thus exiting as output light from the light source device. Meanwhile, a part of the excitation light not having been subjected to frequency conversion is transmitted through the base 100 and reaches the space on the side opposite to the excitation light source 103. Then, the light is collimated by a condenser lens 107, reflected by reflection mirrors 108, 109 and further reflected by the dichroic mirror 110, thus exiting as output light.
Thus, by extracting fluorescence in the space on the same side as the excitation light source 103 when seen from the base 100, the fluorescence extraction efficiency can be improved. Meanwhile, since this light source device is intended to be applied in an image display apparatus, the device is configured to extract also the excitation light and utilize it as blue light, so as to emit light within the whole range of three colors of red, green and blue. In other words, a part of unconverted excitation light having been transmitted through the base 100 is ejected as output light by the optical system composed of the condenser lens 107, the reflection mirrors 108, 109 and the dichroic mirror 110. Thus, this light source device requires such an optical system dedicated to propagating blue light, which results in the complicated device configuration.